1. Field of the Invention
This invention relates to a fluorescent screen such as a fluorescent plate or an intensifying screen used in radiography, and particularly to a fluorescent screen partially having a sensitivity difference and a method of manufacturing the same.
2. Description of the Prior Art
In recent years, there has been a sharp increase in lung cancer, and the necessity of early detection thereof has been emphasized and examinations on a mass basis has occurred. Lung cancer is generally classified into the lung field type, created at the distal end or the like of the bronchus, and the hylar type, created near the thick pipe such as the trachea or the main bronchus. Of these two types, the lung field type lung cancer can be detected by chest radiography heretofore commonly practised, while the hylar type lung cancer can hardly be detected by the popular types of radiography because the hylar portion is photographed while overlapping the spine and heart or pulmonary artery. The hylar portion is thus not depicted when photograpy is effected so that the lung field is of an optimum photographic density.
At the same time, so-called scoliosis, in which the spine is curved, has sharply increased among school children and this, as well as lung cancer, has become a great social problem. Detection of this condition can also be advantageously conducted on a mass basis.
An intensifying screen for converting X-rays into visible light and sensitizing a film by it in radiography, as shown in FIG. 1 of the accompanying drawings, or a fluorescent screen used in the so-called photofluorography in which the X-ray converted into visible light is photographed on a reduced scale or a film through an optical system, as shown in FIG. 2 of the accompanying drawings, generally has phosphor uniformly applied to the whole surface thereof and the luminance thereof is uniform.
In FIG. 1, the X-ray image emitted from an X-ray source 1 and passed through an examinee 2 is formed as a visible image on a film 5 interposed between a frontal intensifying screen 4 and a back intensifying screen 6 through a grid 3.
Also, in FIG. 2, the X-ray image passed through the examinee 2 is formed as a visible image on a fluorescent screen 7 through the grid 3 and is projected onto a film 9 through a reducing optical system 8. Reference numeral 10 designates lead-containing glass.
In recent years, screens in which the sensitivity of a particular portion is improved as seen in Japanese Laid-open Patent Application No. 73400/1981 have been provided with the above-described mass-examination as the object. However, there is the medical practitioners' opinion that in these screens wherein a high sensitivity portion is partially provided, it is clinically inconvenient if the boundary line between the high sensitivity portion when observed as a photograph and the other portion is conspicuous.
As a method for partially improving the sensitivity in an intensifying screen or a fluorescent screen, many possible methods exist. There are methods such as partially increasing the thickness of the phosphor layer and improving the sensitivity, or a method of providing a light-reflecting layer such as a white pigment between the base screen and the phosphor layer and thereby improving the sensitivity of that portion. There are also methods such as of combining phosphors different in luminance, or a method of providing between the base screen and the phosphor layer an absorption layer comprising a colorant having a color such as black, blue or red, reducing the sensitivity of this portion as compared with that of the portion having no absorption layer and thereby providing a sensitivity difference. However, it is difficult to make the boundary between the high sensitivity portion and the low sensitivity portion inconspicuous by these methods. Also, it is difficult to make a number of homogeneous screens matching the desired specification. Japanese Utility Model Application Publication No. 10425/1960 discloses a system whereby printing or the like is effected on a base screen to thereby change the sensitivity, but by the latest trial manufacture, it has been confirmed that even if a base screen is subjected to gradation (continuous variation in density) type printing, which is generally practised, the boundary, between the portion of the base screen on which the printing is not effected and the portion on which the printing has been started, is photographed clearly and this is clinically inconvenient. FIG. 3 of the accompanying drawings shows the reflection density curve of a base screen (reflection density 0.06) subjected to fine dot printing of 200 lines per inch so that the density varies gradually. The highest density is 0.38 in terms of reflection density and the base screen is very smoothly printed with grey ink. A fluorescent screen manufactured by applying a uniform thickness of rare earth phosphor to this base screen was mounted on an X-ray mirror camera, and X-ray was applied with a phantom having a thickness of 10 cm as the object to be photographed and the phantom was photographed on a film. FIG. 4 of the accompanying drawings shows a transmission density curve obtained by scanning the density varying portion obtained on the film, by a micro-photometer. From this Figure, it is seen that the boundary between the portion of the base screen on which printing is not effected and the portion on which printing has been started is sharply varied in density as indicated at A on the transmission curve. This will be reproduced and result in an undesirable portion on a photograph used in diagnosis.